This invention relates to digital carrier transmission, and more particularly to an apparatus and method for detecting digital carrier synchronization problems.
Digital carrier systems are widely used to transmit data and PCM encoded voice signals over telephone lines. The T1 carrier system, for example, transmits 24 channels of 8-bit information formatted in frames--each frame consists of 192 information bit positions, i.e. time slots, and a framing pulse at a transmission rate of 1.544 megaHertz (mHz). Digital carrier signals typically exhibit phase jitter and wander relative to a timing reference clock, even when properly synchronized to the reference clock. Sometimes digital carrier signals become unsynchronized with the timing reference clock and start to slip in phase continuously with respect thereto, ultimately resulting in degradation in the transmission or loss of the transmitted information. Digital carrier data, as distinguished from PCM encoded voice, transmission over telephone lines is particularly vulnerable to loss of information, and thus, to creation of serious errors in the transmitted data.
One technique to analyze the extent of digital carrier signal slippage and to detect synchronization problems is to compare a clock signal derived from the signal under test with a reference clock signal visually on an oscilloscope display synchronized to the reference clock signal. It is difficult to distinguish continuous slippage from jitter and wander in this way. As a result, synchronization problems may go undetected from inspection of the oscilloscope display.